Phospholipase A2 (PLA2) hydrolyses the sn-2 position of cell membrane phospholipids to release arachidonic acid, which is metabolized by 5-lipoxygenase to leukotrienes or by prostaglandin endoperoxide synthase (PGHS) to prostaglandins and thromboxane. Leukotrienes (LT) and prostaglandins (PG) have been implicated in diverse physiologic and pathologic processes. Specifically, LTC4 and PGD2, the principal eicosanoid products of the mast cell, have been implicated in bronchial asthma. We have previously described immediate and delayed phases of eicosanoid generation by mouse mast cells. We have demonstrated that cytosolic PLA2 (cPLA2) is essential for the supply of arachidonic acid in each phase. cPLA2 is also required for eicosanoid generation by mouse macrophages. Nevertheless, there is a body of evidence that cPLA2 requires the co-ordinate action of a low molecular weight, secretory PLA2 (sPLA2), most likely group V sPLA2. Group V sPLA2 is expressed by both mast cells and macrophages. In transfection studies, it coupled to cPLA2 in both immediate and delayed phases of eicosanoid generation. There are no specific inhibitors of group V sPLA2. The hypothesis of this proposal is that group V sPLA2 is essential for eicosanoid generation by mast cells and macrophages in vitro, and in the provision of LTC4 and PGD2 in bronchial asthma. To test this hypothesis we have derived mice with disruption of the gene encoding group V sPLA2. In Specific Aim 1 we will use mast cells and macrophages derived from mice with homozygous disruption of group V sPLA2 to determine its role in immediate and delayed phases of eicosanoid generation in response to diverse stimuli. In Specific Aim 2 we will use mice with deletion of group V sPLA2 to determine its role in inflammatory responses of increasing complexity. We will begin with mast cell-dependent passive cutaneous anaphylaxis. We will proceed to active cutaneous and systemic anaphylaxis, in which group V sPLA2 may additionally contribute to the sensitization process. This will lead to mouse models of bronchial asthma in which group V sPLA2 may participate through diverse mechanisms and diverse cellular processes.